The present invention relates to storage tanks and more particularly to those of the floating roof variety which employ drain valves. In connection with the drainage of floating roof storage tanks, it has been generally known to provide storage roof drain valves which employ floats that float in water and sink in oil. One such valve has been marketed by Sysco Company under the name "Sysco Ball Valve". This particular type of valve generally has been found to be troublesome because of problems experienced with self-starting as well as sticking during operation caused by various valve components which are in sliding contact. Other prior art valves are disclosed in U.S. Pats. Nos. 1,801,172 of Patterson; 2,931,534 of Wiggins; 3,074,587 of Jennings; and 3,491,911 of Fraylick et al. In Patterson the valve is closed by a trigger mechanism activated by means of a float which is lifted by leaking fluid. The Patterson valve automatically shuts off the roof drain outlet when the roof drain line is broken. Wiggins relates to improved emergency drain for a floating roof. It does not function as a normal drain but only under emergency purposes. The Jennings valve automatically opens the roof drain outlet when it rains and shuts when it does not rain. The valve is controlled by means of a diaphragm; its opening and closing are effected by a differential pressure on the diaphragm which is created by the water in the roof drain line and the liquid in the tank. It has deficiencies in that there is some amount of continuous escape and also need for a second tank nozzle for balancing the line, and is relatively unreliable at low roof positions because there is less differential fluid pressure to operate the valve. This differential pressure directly varies with the height of the liquid in the tank. In Fraylick et al the valve will operate generally the same as that of the present invention disclosed by leaking oil which dissolves the oil soluble block which in turn activates the valve opening. In this valve some oil can leak out prior to the valve closing and also the drain line has to be cleaned of any oil contamination.
It is therefore apparent that there is a need for a reliable valve which is basically simple, can be conveniently and easily attached to the drain outlet, and does not permit oil to escape.